Ch 3 Page 29 | David MacKay - Sustainable Energy - Without The Hot Air

3 Cars

For our first chapter on consumption, let’s study that icon of modern civi- lization: the car with a lone person in it.

How much power does a regular car-user consume? Once we know the conversion rates, it’s simple arithmetic:

For the distance travelled per day, let’s use 50 km (30 miles).

For the distance per unit of fuel, also known as the economy of the car, let’s use 33 miles per UK gallon (taken from an advertisement for a family car):

33 miles per imperial gallon ≈ 12 km per litre.

(The symbol ≈ means “is approximately equal to.”)

What about the energy per unit of fuel (also called the calorific value or energy density)? Instead of looking it up, it’s fun to estimate this sort of quantity by a bit of lateral thinking. Automobile fuels (whether diesel or petrol) are all hydrocarbons; and hydrocarbons can also be found on our breakfast table, with the calorific value conveniently written on the side: roughly 8 kWh per kg (figure 3.2). Since we’ve estimated the economy of the car in miles per unit volume of fuel, we need to express the calorific value as an energy per unit volume. To turn our fuel’s “8 kWh per kg” (an energy per unit mass) into an energy per unit volume, we need to know the density of the fuel. What’s the density of butter? Well, butter just floats on water, as do fuel-spills, so its density must be a little less than water’s, which is 1 kg per litre. If we guess a density of 0.8 kg per litre, we obtain a calorific value of:

8 kWh per kg × 0.8 kg per litre ≈ 7 kWh per litre.

Rather than willfully perpetuate an inaccurate estimate, let’s switch to the actual value, for petrol, of 10 kWh per litre.

Congratulations! We’ve made our first estimate of consumption. I’ve dis- played this estimate in the left-hand stack in figure 3.3. The red box’s height represents 40 kWh per day per person.

Figure 3.1. Cars. A red BMW dwarfed by a spaceship from the planet Dorkon. Figure 3.2. Want to know the energy in car fuel? Look at the label on a pack of butter or margarine. The calorific value is 3000 kJ per 100 g, or about 8 kWh per kg. Figure 3.3. Chapter 3’s conclusion: a typical car-driver uses about 40 kWh per day.